1. Field of the Invention
The present invention relates to a liquid crystal display device, which is particularly improved in viewing angle characteristics on a tilt angle in fringing field switching (FFS) and has wide viewing angle characteristics.
2. Description of the Related Art
Liquid crystal display devices have been used in various displays for computers, televisions, and the like because of having excellent characteristics of a small thickness, a light weight, and a low power consumption.
There are various operation modes of the liquid crystal display panels of the liquid crystal display devices. Examples of such operation modes are: a TN (Twisted Nematic) mode in which liquid crystal molecules between two glass substrates are twisted and oriented; an IPS (In-Plane Switching) mode in which an electric field is transversely applied by a pair of two electrodes formed on one glass substrate to rotate liquid crystal molecules; an FFS (Fringing field switching) mode in which the alignment of liquid crystal molecules is controlled by a fringe electric field between common and pixel electrodes formed to be laminated on one glass substrate; and the like. In each of the operation modes of the liquid crystal display panels, the quantity of light passing through a liquid crystal display panel is changed to display by making light from a backlight incident on the liquid crystal display panel and by controlling the alignment of liquid crystal molecules by a voltage.
FIG. 1 is a cross-sectional view that illustrates the structure of a liquid crystal display panel 519 in the FFS mode. The liquid crystal display panel 519 in FIG. 1 is composed of: a TFT substrate 502 and a color filter substrate 504 which are affixed to each other at a kept certain distance; and a liquid crystal layer 505 sandwiched between the two substrates (1999 SID International Symposium Digest of Technical Papers, “A Novel Wide-Viewing-angle Technology: Ultra-Trans View”, (p. 202) (Non Patent Literature 1)).
The TFT substrate 502 is composed of: a thin film transistor and various wirings which are not illustrated; two transparent electrodes 510 and 511 which are laminated to sandwich an insulating film 513 and include ITO and the like; and an oriented film 518 which includes polyimide and the like and is subjected to orientation treatment, on a glass substrate 501. The transparent electrode 511 disposed in a side closer to the liquid crystal layer 505 in the two transparent electrodes 510 and 511 is provided with openings to control the alignment of liquid crystal molecules 517 in the liquid crystal layer 505 by a fringe electric field 525 generated between the two transparent electrodes.
In contrast, in the color filter substrate 504 affixed to the TFT substrate 502, a black matrix which is not illustrated and a color resist which is not illustrated are formed on a glass substrate 501, an overcoat which covers the black matrix and the color resist and is not illustrated is laminated, and an oriented film 518 subjected to orientation treatment in the same manner as in the array substrate is then formed.
The orientation of the liquid crystal molecules 517 in the liquid crystal layer 505 sandwiched between the TFT substrate 502 and the color filter substrate 504 is controlled by the oriented film 518 when no voltage is applied. As the oriented film 518, an organic film such as a polyimide film is typical. In order to control the orientation of the liquid crystal molecules 517 in approximately parallel to a substrate surface, the organic film is subjected to orientation treatment for giving orientation, such as rubbing treatment, irradiation with light such as ultraviolet rays, or ion irradiation. Therefore, the orientation state of the liquid crystal molecules 517 during application of no voltage depends greatly on the orientation treatment and the material of the oriented film.
Examples of parameters indicating the orientation state of the liquid crystal molecules 517 include anchoring energy, a pretilt angle, and the like. The anchoring energy is a measure that represents how the liquid crystal molecules 517 are bound to an interface (oriented film) while the pretilt angle PT is a measure that represents how the liquid crystal molecules 517 tilt with respect to a substrate plane (oriented film surface) as illustrated in FIG. 2. Since the liquid crystal molecules 517 are going to align in the same direction relative to each other, the tilt angle T of the liquid crystal molecules 517 in a bulk of the liquid crystal layer 505 during application of no voltage is in the state of being tilting from the substrate surface due to the pretilt angle PT.
The tilt angle T in the TN mode, the IPS mode, or the FFS mode is approximately parallel to a substrate. However, the pretilt angle PT depends on an orientation treatment condition such as rubbing, the alkyl side or main chain of polyimide used in the oriented film 518, burning temperature, and the like. Therefore, when a tilt angle T in a bulk of liquid crystal layer 505 is measured by a crystal rotation method, tilt is generally found to be at 1 to 10°.
In the FFS mode, increase in the tilt angle T of the liquid crystal molecules 517 results in increase in the black luminance of the liquid crystal display device in a horizontal or vertical direction or a diagonal direction and therefore results in decrease in the contrast of the liquid crystal display device. Therefore, it is necessary to design the tilt angle T of the liquid crystal molecules 517 in the FFS mode to be as low as near zero.
For controlling the tilt angle T, an orientation treatment condition such as rubbing is an important parameter, and a parameter such as the alkyl side or main chain of polyimide used in the oriented film 518 or burning temperature is found to have a great influence. Furthermore, when the tilt angle T approaches zero, the tilt angle T is determined by the main chain of polyimide.
Although it is easy to simply decrease only the tilt angle T, it is necessary for the oriented film 518 to satisfy various product characteristics. Examples of the characteristics required for the oriented film 518 are uniform orientation, durability against rubbing, abnormal orientation due to generated cutting scrap, the uniformity of the film thickness of the oriented film 518, electric characteristics such as visual persistence, and the like. Therefore, there has not been known any method for simultaneously satisfying the low tilt angle T and the characteristics in the oriented film 518 used in the FFS mode.
Then, Japanese Patent Laid-Open No. 2000-356786 (Patent Literature 1) discloses a method for disposing a conductor layer (electrode) on a substrate in an opposite side in a liquid crystal display device in an FFS mode. In the patent publication, there is described a technology in which liquid crystal molecules having a negative dielectric anisotropy are used, an electric field generated between a pixel electrode and a common electrode, which electric field contains many components perpendicular to the substrate, is generated, and the liquid crystal molecules are controlled to be driven in a plane horizontal to the substrate by a synthetic electric field obtained as a result of overlapping the electric field and an electric field generated between the pixel electrode and an electrode disposed on the opposite side substrate. Furthermore, there is described a technology in which an electrode disposed on the opposite side substrate has a constant potential.
In Japanese Patent Laid-Open No. 2001-091974 (Patent Literature 2), the potential of a signal line is changed depending on other display data, an unnecessary electric field is therefore applied between the signal line and a pixel electrode (or a common electrode) in the external side of a pixel, such an electric field controlling electrode as to cover the edge of the signal line is disposed on an opposed substrate in order to improve leakage of light due to the electric field, a voltage with an opposite sign is applied between the electric field controlling electrode and the signal line, and a perpendicular electric field is applied in order to put liquid crystal molecules in a direction perpendicular to the substrate in a region between the signal line and the pixel electrode (or the common electrode) in the external side of the pixel.